When radium is exposed to air, a black coating of nitride rapidly forms. It combines directly with water to form the hydroxide. It reacts with acids to form the commercially important chloride and bromide. The most important property of radium and its compounds is their radioactivity radiotherapy is used in medicine in the treatment of cancer. Mixed with a phosphor such as zinc sulfide, radium compounds are used in luminous paints. Radium is also used as a neutron source (mixed with beryllium) and as a gamma-ray source.

Sixteen isotopes of radium are known, but only radium-226 (half-life 1,599 years), the most stable of the isotopes, is used commercially. It is a product in the radioactive decay series of uranium-238 it is immediately preceded in this series by thorium-230 and followed by radon -222 (a gas formerly called radium emanation). In its radioactive decay radium emits alpha, beta, and gamma rays and also produces heat (about 1,000 calories per gram per year). The curie is a unit of radioactivity defined as that amount of any radioactive substance that has the same disintegration rate as 1 gram of radium-226, i.e., 3.7×10 10 disintegrations per sec. Radium decreases in radioactivity about 1% in 25 years.

Radium is a rare metal. Its compounds are found in uranium ores there is usually about 1 part of radium to 3 million parts of uranium in these ores. Although some radium is obtained from carnotite from Colorado, the chief sources are carnotite from Congo (Kinshasa) and pitchblende from W Canada. Radium is present in all uranium minerals and is widely distributed in small amounts. Radium is usually obtained (with barium impurities) in residues from the production of uranium. It is recovered as the bromide by an involved chemical process. The small amount of the element present in any ore and the difficulty of extraction make it expensive. Radium also is a dangerous material prolonged exposure to even small amounts may cause cancer, anemia, or other disorders. Other radioisotopes (e.g., cobalt-60) are often used in its place when they are less expensive, more powerful, or safer to use.

Radium was discovered in 1898 by Pierre and Marie Curie in pitchblende given them by Austria after the uranium salts had been removed for use in glass manufacture. They had earlier found polonium in a similar sample. Metallic radium was isolated by electrolysis in 1910 by Marie Curie and André Debierne they first formed a mercury-radium amalgam by electrolysis and then removed the mercury by distillation.